The present invention relates to power transistors for radio frequencies, in particular transistors based on SiC and the thermal design thereof.
In the output stages of radio frequency transmitters of communication systems power amplifiers are used. In the power amplifiers various active devices are used. For low output levels devices based on semiconductors are used and for high power output, such as above 1 kW, vacuum tubes or other special amplification means are used. Thus, travelling wave tubes (TWTs), klystrons, can be used for power levels up to 10 MW.
The semiconductor devices include basically different types of transistors. Transistors suitable for high frequencies were first fabricated based on germanium but were then replaced by bipolar transistors based on silicon which have since been the dominating devices used for power amplification at radio frequencies. In mobile telecommunication systems bipolar silicon-based transistors are presently used in the output amplifiers of base stations and they have a good performance up to at least 2 GHz. Thus they have a good stability, are easily available/fabricated and have a relatively low cost.
Other semiconductor devices also used for RF power amplification include MESFETs based on GaAs and the recently developed laterally diffused MOS-transistors (LD-MOSes). Generally, there is still a great need for improved or new devices to be used in power amplifiers because of the expanding use of telecommunications, also for high frequencies.
Power transistors are especially designed to deliver a high output power and to have a high gain. The manufacturing processes, device parameters, layouts and packages have been carefully studied and tuned to achieve this. The transistors have to fulfil a multitude of requirements as to breakdown voltage, DC gain or transconductance, capacitance values, RF gain, ruggedness, noise figure, input/output impedance, distortion, etc. The frequencies for which the transistors are designed range from several hundred MHz up to and into the microwave region. When designing transistors for increasing output power, from the output power level of 1 W special considerations have to made and this power level may be taken as a rough lower limit of the output power provided by a transistor which can be called a power transistor. Transistors for lower output power can be called xe2x80x9cnormal transistorsxe2x80x9d and are used for example for small signal processing, pulse switching, etc., the output power levels being lower than about 1 W. For power applications, usually only one transistor of n-channel type built on single die or chip is used. However, when the power required is very high, such as up to hundreds of watts or even up to kilowatts, this power is achieved by arranging a plurality of parallel transistor cells on a single die or even using a multitude of separate dies comprises in a single package. Packages containing such power components often have large gold-plated heat sinks to conduct the heat generated by the components.
Transistors based on silicon carbide (SiC) have recently been studies as a possible alternative of transistors based both on Si and GaAs for power applications at GHz frequencies. The unique properties of SiC include a high electric break-down field of e.g. about 4xc2x7106V/cm, a high saturated electron drift velocity of e.g. about 2xc2x7107cm/s and a high thermal conductivity of e.g. about 4.9 W/cmxc2x7K. Therefore, devices based on SiC have been predicted to be capable of handling much higher power densities compared to those handled by the other mentioned transistor types, and they can also operate at much higher temperatures owing to the superior properties of the SiC-material. This has been experimentally demonstrated. SiC devices preferably are operated at very high supply voltages of 48 V or more in order to fully exploit the advantages of the devices.
One of the problems associated with currently used semiconductor transistors manufactured based on Si or GaAs is the limited temperature at which the performance of such transistors starts to degrade. Normally, Si-transistors are not operated above 150xc2x0 C. temperature of the active junction. This temperature sets the limit on the possible temperature of the heat sink and on the ambient temperature, which for a radio base station (RBS) normally is limited to 70xc2x0 C. and 60xc2x0 C., respectively. However, it would be very advantageous to allow an increase of both the ambient temperature and the temperature of the heat sink to eliminate external cooling such as by air conditioning. Then, it could be possible to mount power amplifiers directly on the antennas of such a station where the temperature in some cases in hot climates can exceed the above mentioned temperatures. Transistors based on SiC can operate at higher temperatures than transistors based on Si, and the heat conductivity of SiC is three times that of Si, which allows heat to be conducted away from Sic-devices much more efficiently. Thus, SiC-based amplifiers are well-suited to be mounted at places where the ambient temperatures are high. However, there is one particular problem associated with SiC-devices. In order to achieve a high performance, SiC-devices must operate at power densities which are several times, e.g., about 3-20 , higher than devices based on Si. Since power amplifiers used for radio frequencies are normally operated as Class A or Class AB amplifiers, 40-60% of the total input power is dissipated in the SiC-transistor itself. Thus SiC-transistors will dissipate much more power per unit area than Si-transistors. This high power heats the SiC-transistors and to high temperatures and degrades its electrical characteristics. Also, the heat conduction depends on temperature and decreases with increasing temperature. Hence, the much higher power dissipation density will offset the improved power handling capability of SiC-transistors provided that no particular measures are made to make the heat transport from the devices more efficient.
Field effect transistors for high power applications are e.g. described in the published European patent application No. 0 518 683.
It is an object of the present invention to provide a transistor based on SiC with a high output power which still has a moderate temperature.
The problem, which the invention solves, is how to construct a transistor based on SiC that operates at high output power and does not require special cooling measures.
A field effect transistor, such as a MESFET, is made on a chip comprising a SiC-substrate. The transistor includes a plurality of densely stacked parallel transistor cells occupying a rectangular active area. Each transistor cells has parallel strip-shaped regions forming the electrodes and active areas of the cell, and each inner cell shares its drain and sources electrodes with neighboring cells. The active area has a very elongated shape, and specifically, it should have a width not larger than substantially 50 xcexcm, in order to give a good power dissipation allowing an electrical high power in the operation of the transistor. In the active area, all the transistor cells have their strip-shaped regions located in parallel to the short sides of the rectangular area, the cells thus being relatively very short considering the length of the active area. Each cell has a length not larger than substantially 50 xcexcm. The distances from the long sides of the rectangular area to the edges of the chip should be at least 50% and preferably 60% of the thickness of the chip to allow a good thermal flow out of the active rectangular area.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.